1. Field of the Invention
The present invention generally relates to multi-color printing devices and, more particularly, to a multi-color printing device which saves printing film by only applying a particular color to the printing medium when that color is needed for printing on the medium.
2. Discussion of the Related Art
Currently available thermal transfer printing heads generally comprise a plurality of thermal elements. The heating elements are generally provided on one side of a flat substrate of ceramic or like material along with the requisite electronic circuitry for controlling the activation of the heating elements.
The thermal transfer printing head is typically used in conjunction with a roller platen assembly and ink transfer printing film or ribbon which carries a thermally transferable printing ink. During printing, a web of material to be printed are oriented between the thermal elements of the printing head and the roller platen of the roller platen assembly such that the printing ribbon is adjacent to the print head and the material to be printed is adjacent to the roller platen. The roller platen and printing head are moved against each other so that the printing ribbon and the material to be printed are pressed against the printing head such that when selected thermal elements of the printing head are heated, ink from the ink transfer printing ribbon is transferred to the surface of the material.
Typical thermal transfer printers can include one or more printing heads, each of which are capable of printing in a different color. If more than one printing head is employed, the printing heads are generally arranged in line with each other, with each printing head having a roller platen assembly associated therewith.
In typical multi-color thermal transfer printers, each printing head and roller platen assembly is associated with a printing film of a particular color. During the printing process, the media to be printed is passed between the head and roller platen of each roller platen assembly along with the printing film of each assembly. For simplicity, the printing media referred to in this application is a series of labels serially attached on a continuous web. When a particular color is to be printed on a label, the printing head is activated, thus transferring the ink from the film to the label.
In one type of prior art thermal transfer printer, the film is wound off of a supply roll and through the roller platen assembly at the same rate that the label web is fed through the assembly. While this ensures that a fresh portion of printing film is always in contact with the label when the printing head is activated, it also wastes a great deal of the film, especially if a particular color is used sparingly for a particular print job. Since the label web and the printing films are fed through the printing apparatus at a 1:1 ratio, for each length of a label, an identical length of printing film is used. Therefore, in a system which uses four printer assemblies to print four colors on a label, for each label printed, four times as much film is used.
In an attempt to reduce the amount of film used in this printing process, prior art printing devices have been developed in which the printing head of a particular printing assembly is lowered against the roller platen only when the particular color associated with that printing assembly is to be applied to the printing media. When the color is not being applied, the printing head is raised away from the roller platen. Therefore, the printing film is only fed through the printing assembly while that particular color is being printed. While this type of device effectively reduces the amount of film used, it presents other problems. Specifically, when the printing head is mechanically urged downward onto the roller platen, by, for example, a motor-powered drive mechanism, the impact of the printing head against the roller platen causes printing irregularities, such as smudging, blurring and color darkness inconsistencies. Furthermore, the impact of the printing head against the roller platen shortens the effective life of the printing head.
Therefore, what is needed is a printing mechanism which conserves the amount of printing film used for a particular printing process, while also reducing printing errors which can occur in the prior art printing mechanisms.